The use of ethylene-containing aqueous latexes is of significant importance. Generally these latexes are produced at elevated pressures thus necessitating the use of special pressure resistant equipment. Of particular interest are the vinyl chloride-ethylene and vinyl acetate-ethylene latexes; in many instances in combination with other comonomers. Over the decades the technology has developed extensively from the early efforts exemplified, for example, in U.S. Pat. No. 2,497,291 to the more refined procedures employed in the production of such latexes in use today.
In order to achieve a desired instantaneous polymeric chemical composition, semi-continuous batch emulsion copolymerization reactions have long been carried out employing vinyl chloride, vinyl acetate or styrene in significant concentration in the monomers mixture (i.e. from about 40 weight percent or more). In many instances the reaction is carried out at atmospheric pressure with the mixed monomers fed into the reactor during the course of the polymerization, the objective being to hold the level of free monomers present in the system at as low a level as possible (relatively starved conditions) and force the copolymer produced to have a composition as similar to the monomers mixture feed as possible. The systems may or may not be below the saturation level of the monomers in the aqueous phase. Generally there is sufficient free radical initiator present to cause the monomers to react as rapidly as they are added, hence, the rate of polymerization is essentially equal to the rate of monomers mixture feed introduced to the reactor. If for any number of reasons the rate of polymerization is slower than the rate of monomers feed, the free monomers concentration will build, often with undesirable results; however, since so many of these systems are reacted at atmospheric pressure, no signal is normally generated to signify a slow increase in free monomer concentration in the reactor. Conversely, a semi-continuous batch emulsion copolymerization can be carried out by the utilization or addition of more and more initiator in order to make the polymerization reaction more and more starved; this, however, is not desirable because excess initiator will damage the latex properties and/or result in the formation of undesirable low molecular weight copolymer. Again, the degree of starvation is not normally known.
An example of a starved, semi-continuous polymerization reaction may be seen with reference to U.S. Pat. No. 3,423,353 for vinyl acetate based polymerizations in which a constant composition of copolymer is formed throughout the run by a continuous feed of a single monomers mixture whose compositional content remains unchanged during the feed. The feed is introduced continuously and is not controlled by either the pressure or the temperature in the reactor.
The procedure disclosed in U.S. Pat. Nos. 3,804,881, 4,039,500 and 4,111,876 differ from the above in a significant manner. In these references, the compositional content of the monomers present in the feed added to the reactor is continuously changing during the period that the monomers mixture is added to the reactor. This continually changing feed mixture is continuously added to the reactor for immediate polymerization and the copolymer produced, since there is essentially instantaneous reaction, is continually changing in compositional content and corresponding to the changing compositional content of the feed. Again, while the monomers feed is continuously added to the reactor, its rate of feed is not controlled by either the pressure or the temperature in the reactor but it is initially predetermined and maintained at a set, constant rate by the operator. In the three patents identified in this paragraph there is disclosed a process for producing non-uniform polymers by the technique of varying the compositional content of the monomers mixture added to the reactor by the use of primary and secondary feeds. The secondary feeds are added to the primary feeds to continually change the compositional content of the primary feed and this continually changing primary feed is essentially simultaneously added to the reactor and polymerized under such conditions that there is essentially instantaneous polymerization. The feed rate to the reactor is not controlled by pressure variations observed in the reactor, pre-set, predetermined uniform rates of addition are shown in each of the examples and there is no indication in the references that the rates of addition during a reaction vary or change in response to variations in the reactor pressure. Nor is there any indication of degree of saturation of the aqueous system in these continuous feed polymerization processes.
Other emulsion processes for producing vinyl chloride-ethylene or vinyl acetate-ethylene based copolymers generally operate either as a continuous process or as a semi-continuous batch process with ethylene overpressure in the reactor. The continuous process is exemplified by U.S. Pat. No. 4,035,329 that employs a series of filled reactors. In the process disclosed in this patent the reactors are initially filled with a preformed polymerized dispersion corresponding essentially to the dispersion that is to be produced and the temperature and pressure in the reaction system is set at the desired values. The reactant streams, including a preformed mixture of the monomers, are then added to the first reactor and passes through the entire reactor system. Simultaneously, the finished product leaves the reactor system in the same proportion or amounts as the reactant streams that were added; at all times the reaction system is completely full of liquids. Under the conditions employed, pressure cannot be used to measure the amount of monomers concentration, it is used basically as a means of controlling the reaction and maintaining full reactors. The pressure was not used to control the feed rate of reactants into the reactor.
A typical semi-continuous emulsion process for producing copolymers is that which is disclosed in U.S. Pat. No. 4,123,405, which relates to a process whereby monomer is initially charged to a reactor which is then pressurized with ethylene. Then a mixture of monomers is added at a fixed rate of feed while maintaining a fixed pressure; however, the supply of ethylene was stopped during this feed. In this reference there is no control over the ratio of ethylene in the monomers mixture and there is no teaching of control of rate of feed by the pressure in the reactor. The process disclosed teaches the feeding of a mixture at a fixed rate independent of the pressure in the reactor. One using such a process is faced with two issues. Initially, if the vinyl acetate/vinyl chloride content is very low, a low molecular weight copolymer having inferior mechanical properties is produced. Secondly, since the ethylene is not ratioed to the other monomers the entrance of the ethylene into the polymerization reaction requires diffusion from the vapor phase into the aqueous phase at a significant rate and exterior factors such as agitation and latex viscosity can significantly disrupt the flow of ethylene to the site of the polymerization and result in a reduced amount of ethylene in the copolymer produced.
As can be seen, the known art clearly demonstrates some of the problems that are still prevalent and demonstrates the need for further improvements that would enable one to carry out the process in such manner that there was improved control of the free monomers content in the reactor as well as control of the monomers content in the polymer itself. Further, if such improvements could be attained at significantly lower polymerization pressures, major savings in capitol equipment costs and investment could be attained.